1. Field of the Invention
The present invention relates to a high voltage vacuum insulating container used for a vacuum switch tube an electron tube or the like.
2. Discussion of the Background
FIG. 6 is a cross-sectional view of an X-ray tube disclosed in, for instance, Japanese Unexamined Patent Publication No 36735/1982, and FIG. 7 is an enlarged view showing a part A in FIG. 6. In FIGS. 6 and 7, a reference numeral 1 designates an insulating tube, a numeral 2 designates a sealing metal member, numerals 3 and 4 designate electrodes and numerals 3A and 4A designate terminals for the electrodes. A metal ring 11 is attached to both ends of the insulating tube so as to be close to the outer circumference of the insulating tube 1, and the sealing metal member has its free end curved with a large radius of curvature. The insulating tube 1, the sealing metal member 2 and the metal ring 11 constitute a high voltage vacuum insulating container 20.
The function of the above-mentioned high voltage vacuum insulating container will be described. Generally, the high voltage vacuum insulating container 20 used for a device such as an electron tube, a vacuum switch tube or the like maintains the electrodes 3, 4 in a vacuum condition and isolates electrically the one of the electrodes 3, 4 from the other. As a material for the insulating tube 1 which constitutes the main body of the high voltage vacuum insulating container 20, glass or ceramics is usually used. In particular, ceramics having an excellent strength is widely used. When such ceramic material is used for the insulating tube 1, it is necessary to sealingly attach the sealing metal members 2 to the insulating tube 1. The attaching of the sealing metal members 2 has been conducted by forming a metallized layer 5 such as molybdenum, manganese or the like at the ceramic side and the sealing metal members 2 are attached to the metallized layers 5 by soldering.
The conventional high voltage vacuum insulating container 20 having the above-mentioned construction had a problem that when a high voltage is applied to the container, an electric field is concentrated to a metallized layer on the ceramic tube or a soldered portion to thereby produce an electric discharge along the outer surface of the ceramic tube. In order to solve such problem, the metal ring 11 was proposed. The metal ring 11 is to moderate the concentration of an electric field near the metallized layer 5, the soldered portion or a joint portion therebetween because the metal ring 11 having a bent portion bent with a large radius of curvature is arranged in an annular form in the vicinity of the outer circumference of the both ends of the ceramic insulating tube. Thus, an electric discharge caused along the outer circumference of the ceramic tube was suppressed.
However, in the high voltage vacuum insulating container as constructed above, when a further high voltage was applied across the electrodes, there was found a creeping discharge from the metal ring 11 to the insulating tube as indicated by a reference numeral 6 in FIG. 8. The creeping discharge results because the metal ring 11 is disposed in the vicinity of the insulating tube 1 and the intensity of an electric field increases at the place including the metal ring 11 and the surface of the insulating tube 1 which are adjacent to each other. In order to increase a creeping discharge voltage, the metal ring 11 has to have a large radius of curvature, which results in the manufacture of a high voltage vacuum insulating container with a metal ring 11 having a large outer diameter. This is contrary to a demand of miniaturization of a container for an electron tube, a vacuum switch tube or the like.